CT scanners have become an established technique for acquiring essentially planar cross-sectional images of a live body anatomy or the interior of an inanimate object. There are three essential features sought in high-quality CT scanners: (1) high spatial resolution; (2) high-contrast resolution for tissue differentiation; and (3) high scan speed to minimize blurring from patient motion and to perform dynamic studies in which several scans are acquired in rapid succession. High spatial resolution is generally characteristic of images acquired in translate-rotate CT scanners whereas high scan speeds are generally characteristic of rotate-rotate CT scanners.
The intrinsic spatial resolution of a CT scanner is determined primarily by two factors: (1) effective beam width at the center of the object, and (2) sampling frequency. The effective beam width is a function of the focal spot size, the detector aperture width, and the magnification factor (defined as the X-ray tube-object separation v. the X-ray tube-detector separation); this is true regardless of whether the scanner is operated in the translate-rotate or rotate-rotate mode. Assuming the effective beam width has been optimized, the sampling frequency becomes all important. As to sampling frequency, the difference between translate-rotate and rotate-rotate acquired data is critical.
In rotate-rotate scanners, the sampling frequency, as well as the effective detector aperture, are limited by the size of detectors provided. This is due to the intrinsic geometry of a rotate-rotate scanner in which the X-ray source and the bank of detectors are fixed in relation to one another, and are both rotated about the object. As a result, the geometry of rotate-rotate scanners restricts the smallest possible sampling distance to the distance between the detectors, and the sampling frequency to once per beam width. According to the Nyquist criterion, however, the sampling frequency should be at least twice as great, i.e., two or more measurements per beam width. Because the geometry of the rotate-rotate scanner does not satisfy the Nyquist criterion, image-degrading aliasing artifacts may be caused by high contrast, high spatial frequency structures in the image. In order to avoid aliasing artifacts, the data must be pre-filtered by combining measurements in adjacent detector channels to attenuate the high spatial frequencies having a period less than two beam widths. In this manner, a new beam width is effected, which is twice as large as the actual beam width, so that the Nyquist criterion is satisfied. Thus, the intrinsic spatial resolution capability of the rotate-rotate scanner, as measured by its beam width, must be degraded by a factor of two to prevent aliasing artifacts.
By contrast, in a translate-rotate scanner, the gantry to which the X-ray tube and detectors are fixed may be indexed in increments smaller than or equal to half the beam width, satisfying the Nyquist criterion. Thus, aliasing artifacts are eliminated while preserving the intrinsic spatial resolution of the system.
Furthermore, in rotate-rotate scanners, the above-described lack of flexibility in adjusting the sampling frequency would make post-patient collimation to reduce the beam width futile because the distance between the detectors is constant and narrow beams would not improve the ultimate spatial resolution beyond the limit set by the sampling frequency.
By comparison, in translate-rotate scanners, post-patient collimation may be used to reduce beam width and to improve spatial resolution, because the gantry may be indexed in correspondingly smaller increments to maintain a sampling frequency of at least twice per beam width.
To compensate for limitations in sampling frequency imposed by the one-ray-per-detector relationship inherent in conventional rotate-rotate scanners, some rotate-rotate scanners use a technique of offsetting the center of rotation to simulate an increase in the sampling rate. Using this technique, if the center of rotation of the gantry (i.e., its iso-center) is offset by a distance equal to one-fourth the effective beam width at the iso-center, two views taken 180.degree. apart will be shifted by one-half of the detector pitch. It can be seen that, using this technique, after the gantry has rotated 180.degree., the rays from the diametrically opposed views interleave such that the sampling density is effectively doubled and spatial resolution is improved. However, this technique only works in the ideal case where there is no patient motion. If the object to be scanned moves by a fraction of a millimeter during the few seconds required for gantry rotation, registration will be lost, and proper interleaving of the views will no longer be achieved. This can introduce aliasing artifacts which degrade the image. Thus, although this technique simulates doubling the sampling frequency at the center of the object, it does not totally absolve rotate-rotate type scanners from the above-described deficiencies resulting from limited sampling frequencies.
Another method of increasing the sampling density is to collect data from the detectors in a given position, and then to shift the detectors laterally (or rotate them about the iso-center) by half of the detector-to-detector pitch while the X-ray source is in the same position, and collect additional data; this results in interleaving of the data collected in the first 180.degree. rotation with that collected in the second 180.degree. rotation such that the sampling frequency is effectively doubled. These data are then processed in the usual way (i.e., filter and back-projection) to form a CT image. However, the mechanics of moving the detectors but not the X-ray source during a scan as described above is inconvenient in rotate-rotate scanners, and would operate to defeat the advantage of simple mechanics which characterizes rotate-rotate CT scanners.
U.S. Pat. No. 4,149,079 discloses a system for increasing data density to obtain a more accurate reconstruction in a system having a reduced detector array, i.e., a system in which the apical angle of the fan beam is less than the apical angle of the reconstruction circle. This patent provides for either rotating or linearly displacing the fan beam relative to the fixed center of the reconstruction circle to obtain a second data set after a first data set has been obtained during a complete rotation. This system is, therefore, disadvantageous in that it requires two separate rotations and also mechanical means for shifting the fan beam.
U.S. Pat. No. 4,266,136 discloses a CT apparatus which also uses a reduced detector array. The source emits a fan beam of radiation having an apical angle which subtends less than the diameter of the reconstruction circle so that only one-half of the object slice is irradiated at any given time. Processing means convert the data produced by the detectors into parallelized profile signals suitable for processing by a conventional reconstruction algorithm. This system is disadvantageous in that the acquired data density is insufficient to satisfy the Nyquist criterion and hence poor reconstructed images will be provided thereby.
These above-described prohibitive sampling limitations which are present with rotate-rotate scanners have led to the development of a modified rotate-stationary scanner having a stationary array of detectors. In such systems, a complete circle of detectors is rigidly mounted around the patient area. The X-ray source is located inside or outside the detector area, and data is acquired as the X-ray source is rotated. Although rotate-stationary systems having stationary detectors achieve flexibility in sampling, they create new limitations so that, in the end, their intrinsic spatial resolution and overall clinical performance roughly equal that of the original rotate-rotate arrangement. The most notable problem with rotate-stationary systems is efficiency; i.e., they are costly due to the large number of detectors required. In addition, rotate-stationary systems have structural difficulty in eliminating scatter radiation and associated high background noise; this results in poor contrast resolution. Further, the common rotate-stationary design, which has the X-ray source mounted inside the ring of detectors, is burdened by the difficulty of optimizing the tube-object v. object-detector separation because both the X-ray source and the object must be confined within a detector ring which should be kept as small as possible so that the number of detectors does not become excessive. Another disadvantage in rotate-stationary systems is increased skin dose to the patient due to the short tube-object distance. These problems are severe enough to have prompted the development of a scanner in which the X-ray source rotates around the object outside the detector ring to optimize the distance between the tube, the object and the detectors. Such systems, however, are burdened by excessive mechanical complexities because, in order to allow the unimpeded beams to fall on the detectors on the opposite side of the scanned object, the detectors closest to the tube must be moved out of the field of radiation while the tube rotates. This is accomplished by nutating the detector ring.
It is therefore an object of the present invention to provide a new and improved computerized tomography method and apparatus which substantially overcomes the above-described deficiencies in the prior art.